In two hereditary syndromes of the dog, Mullerian ducts persist in the presence of testicular tissue: XX sex reversal and Persistent Mullerian Duct Syndrome (PMDS). The objective of this project is to determine whether the lack of Mullerian duct regression in these animal models of human intersexuality is the result of a deficiency in Mullerian Inhibiting Substance (MIS). If so, these will be the first defined genetic defects in the as yet poorly understood MIS system. Dogs with XX sex reversal and PMDS will be produced by breeding known carriers. The MIS activity of gonads from affected pups will be compared with that of gonads from normal male littermates using a graded organ culture assay for MIS. Results from MIS assays will be examined with respect to gonadal structure, particularly the morphology of Sertoli cells, using light and electron microscopy.